The present invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for fabricating a silicon wafer. In more detail, the present invention relates to an epitaxial silicon wafer and a method for fabricating the same.
Silicon, generally used as a wafer material, is an element naturally occupying approximately 28% of the surface of the earth. Silicon exists in forms of an oxidized substance (quartzite) or a silicate. Powder form silicon of approximately 98% purity, often referred to as metalloid silicon, may be obtained by melting quartzite, which is the chief element of silica, with coke in an electric furnace and then performing a chemical treatment. Polycrystalline silicon of approximately 99% purity may be obtained by transforming the powder form silicon into gas form silicon and then performing a thermal treatment. Since the silicon wafer used during an integrated circuit fabrication is generally required to be mono-crystalline, the polycrystalline silicon is transformed into the mono-crystalline silicon using a physical refining method. The czochralski method is a general method for transforming the polycrystalline silicon into the mono-crystalline silicon.
The czochralski method is briefly described below. A high-purity silicon melted solution in a silica melting pot is maintained at a temperature slightly higher than the melting point by a high frequency inductive heating. In order to grow the mono-crystalline silicon, a piece of mono-crystalline silicon, generally referred to as a seed-crystal, at an upper portion of a shaft is brought in contact with a surface of the liquid, and the shaft is pulled upward while rotating at a speed of approximately 50 mm to 100 mm per hour. The silicon solution grows with the same crystal orientation as the seed crystal, forming a cylinder type silicon mass referred to as an ingot.
The cylinder type ingot grown by the czochralski method is thinly cut in a shape of a disc using a cutter, and the surface is then polished using a chemical mechanical method to form a thin wafer. At this time, the type of the wafer is determined by the type and amount of added impurities. The wafer becomes an N-type wafer if N-type impurities are added such as phosphorus (P) or arsenic (As) which are materials of the group 5 in the periodic table. The wafer becomes a P-type wafer if P-type impurities are added such as boron (B) which is a material of the group 3 of the periodic table. The impurities may be evenly distributed over the entire silicon wafer. A resistance value of the substrate depends on the concentration of the impurities.
Meanwhile, a process for forming another high-purity crystalline layer according to the crystal orientation at the surface of the mono-crystalline silicon wafer grown by the czochralski method is referred to as an epitaxial growth method or an epitaxial method. The layer formed using the epitaxial method is referred to as an epitaxial layer or an epi-layer.
The epitaxial method is performed in two steps while the silicon wafer is loaded into a deposition chamber and on a susceptor. The first step includes pre-baking by supplying a cleaning gas such as a mixed gas of hydrogen or hydrogen/hydrogen chloride acid at a temperature of approximately 1,150° C. on the surface of the silicon wafer, and then cleaning the surface of the silicon wafer. Native oxide formed over the surface of the silicon wafer is substantially removed in a manner that an epitaxial silicon layer evenly grows continuously over the surface. The second step includes supplying a silicon vapor source such as silane or trichlorosilane on the surface of the silicon wafer at a temperature of approximately 1,000° C. or higher to form and epitaxially grow a silicon layer over the surface.
The epitaxial silicon layer grown by the epitaxial method has the following limitation. While the pre-baking at a high temperature and the epitaxial growth method are performed, impurity atoms such as boron or phosphorus are discharged through the other side of the silicon wafer. To reduce the discharge of the impurity atoms, impurities are implanted into the mono-crystalline silicon wafer grown by the czochralski method, and the other side of the mono-crystalline silicon wafer is sealed by an oxide layer or a polysilicon layer using a chemical mechanical deposition (CVD) method. Such process is referred to as a back seal process, and the resulting layer is referred to as a back seal layer.
Accordingly, the wafer fabrication cost may increase during the typical epitaxial silicon wafer fabrication method because the back seal layer is generally required to be formed on the other side of the mono-crystalline silicon wafer highly doped by the back seal process, that is, a bulk wafer.